\documentclass[tikz,convert=pdf2svg]{standalone}
\usepackage{tikz}
\begin{document}
    \begin{tikzpicture}
        \node {root}
            child {node {a1}}
            child {node {a2}
                child {node {b1}}
                child {node {b2}}}
            child {node {a3}};
    \end{tikzpicture}
\end{document}